Froth flotation has been practiced for over a century to separate mineral particles on the basis of differences in chemical composition. Early examples include the separation of sulfide minerals from oxide gangue by forming the ore into an aqueous pulp, adding a xanthate collector reagent which selectively coats one or more desired sulfide minerals from gangue, adding a frother, and subjecting the reagentized ore pulp to aeration by various means. A froth product which is a concentrate of the sulfide minerals which report as a froth which is then separated from the nonfloated gangue-rich aqueous pulp.
Froth flotation was most readily implemented when it was applied to sulfide minerals, especially those of relatively large particle size, for example, larger than 10 microns. A subsequent development was the adaptation to the selective flotation of so-called "oxidized" minerals such as cassiterite, fluorspar, scheelite and titania from other oxidized minerals using anionic (negatively charged) collectors, such as fatty acids or modified fatty acids, sometimes augmented by reagents to control selectivity and/or to control frothing. These processes were also more difficult to implement when the ore in the pulp was very finely mineralized or if the pulp contained slimes.
In all but the coarser particle size fractions of typical kaolin crudes, most of the particles are finer than 5 microns. In fact, the kaolin in the crudes usually have an average particle size (as determined by conventional sedimentation techniques) below 2 microns and frequently have a significant content of particles in the submicron-size range. Furthermore, the colored impurities (principally iron-bearing titanias) are present in small amounts, e.g., 1-5% of the weight of a degritted crude and in the form of very fine particles. Thus one of the greatest challenges in the art of froth flotation was the selective flotation of colored impurities indigenous to most kaolin clays from the kaolin particles.
The successful large-scale flotation purification of kaolins awaited the development in the 1960's of so-called "Ultraflotation", U.S. Pat. No. 2,990,958, the teachings of which are incorporated herein by cross-reference. The process achieved the flotation of colored titaniferous impurities from fine particle-size fractions of kaolin crude by utilizing several modifications of conventional froth flotation technology, including the use of a clay dispersant anionic, collector mixture of fatty acids and rosin acids), addition of "auxiliary," or carrier, mineral particles (calcite) amenable to flotation with an anionic collector, and oils to control frothing. In carrying out the process, the impurities and the calcite were selectively coated with the anionic collector. When the ore pulp was aerated these minerals reported in the froth which was separated on a continuous basis from the remainder, a dispersed pulp of purified kaolin was recovered. The purified clay was further processed by bleaching, filtration washing, etc. In preferred embodiment, the kaolin pulp was first degritted to remove gross particles, generally larger than 325 mesh. Following this the degritted pulp was separated into a fine and coarse particle size fraction, the fine fraction, for example a fraction 80% or 90% by weight finer than 2 microns, being the one which was "conditioned" with the collector and "carrier." The pulp thus conditioned was charged to the flotation cells where other reagents were added. A bank of flotation cells were used in the process, as is conventional in ore flotation, with the froth from one cell being diluted with water to facilitate transfer to the next cell, etc. and the machine discharge products (nonfloated material) being combined to maximize recovery of purified kaolin. The froth product was transferred to an appropriate waste disposal site.
Subsequent efforts to beneficiate kaolin clay by froth flotation include TREP, a flotation operation that can be applied to whole (unfractioned kaolin crudes) and can be operated at higher solids than Ultraflotation. TREP features the use of an oleic acid collector with a calcium salt such as calcium chloride in flotation cells in which air is introduced into a recycle stream of beneficiated clay. In this case, as in Ultraflotation, the concept of using an anionic organic collector for colored impurities such as titaniferous matter, followed by aeration and flotation is applied. See U.S. Pat. No. 4,472,271, the teachings of which are incorporated here by cross-reference.
Another variant of the anionic froth flotation purification of kaolin clay is described in U.S. Pat. No. 3,979,282 which includes an example for the removal of colored tournaline from a clay.
While many advances have been made in these schemes for removing colored impurities from kaolin clay, sometimes referred to as "china clay," most of the developments have adhered to the concept of using an anionic collector to selectivity float impurities and aeration and flotation to effect the separation. Exceptions are high intensity magnetic separation procedures, now widely used in industry, frequently in combination with froth flotation. Also, schemes have been proposed to selectively flocculate impurities from kaolin or to selectively flocculate kaolin from impurities with high molecular weight charged polymers such as polyacrylamide, followed by physical separation of flocculated matter from dispersed particles. Also, it has been proposed to permit an anionically conditioned pulp of impure kaolin to settle quiescently, whereby aeration introduced during conditioning causes impurities to float on a pulp of purified clay. The floated impurities are skimmed from the surface of the pulp of purified kaolin. See U.S. Pat. No. 3,670,883.
None of these procedures has met the widespread acceptance of froth flotation and, in the case of magnetic purification, the technique does not remove titaniferous and other very weakly paramagnetic impurities to the extent achievable by froth flotation. Thus, the froth flotation method for purifying kaolins is one of the largest commercial industrial mineral beneficiation operations in existence.
Froth flotation of kaolin is not without drawback, especially in the present industrial climate in which large scale industrial equipment such as float cells are expensive and wastage of valuable mineral resources through processing loss is of great commercial significance. Further, production of by-product waste streams must be minimized for environmental reasons. Thus, kaolin flotation as presently practiced, even in advanced modes, results in the recovery of beneficiated kaolin in the form of relatively dilute effluents, typically about 13 to 18% solids, depending upon grade, in the case of Ultraflotation and about 30 to 35% solids in the case of TREP. This means that water must be removed from the pulp of dispersed purified kaolin before further processing such as bleaching is carried out. It would be highly advantageous to achieve the high level of titania removal achievable by froth flotation while generating more concentrated streams of kaolin products. Among other obvious benefits, prolonged aging in settling tanks would be avoided, thus minimizing the amount of undesirable airborne quartz dust introduced into the clay during settling.
While there has been ongoing effort by practitioners of the technology to avoid losses of kaolin with the floated impurities, this has been unavoidable in large scale commercial operations. Thus, typical kaolin losses in Ultraflotation are about 8%, based on the weight of kaolin feed to the conditioners and are about 10% in TREP.
Kaolin products, especially premium grades such as those necessitating beneficiation, are supplied as grades, depending upon particle size and brightness criteria. Kaolin clays are polydispersed in the sense that the particles are not composed of particles of a single particle size. If they were, impurities could readily be separated from the kaolin by gravity, provided the impurities had different settling velocities. However, not only are kaolins composed of particles of widely different particle size ranges, e.g., particles as fine as 0.2 microns up to 10 microns, but the distribution of the impurities is such that Stokes law, as applied to gravity settling or even high speed centrifuges, will not separate titaniferous impurities in most kaolins to the extent achievable by froth flotation when applied to simple dispersed kaolin pulps. Thus, it is conventional practice to centrifuge kaolin feed before it is charged to flotation equipment, as in Ultraflotation, or after flotation, as in TREP to recover kaolin fractions of various sizes. The primary role of centrifugation in these operations is to produce grades of desired particle size and not primarily to purify the kaolin.
A related challenge to the kaolin industry is the physical recovery of valuable kaolin from a flotation waste before the waste is impounded in an environmentally acceptable manner. The kaolin should be recovered as a grade of desired particle size. This not only improves the recovery of valuable kaolin but reduces the volume of waste that must be handled. Waste includes titania and/or other impurities as well as organic and inorganic flotation reagents. In the case of Ultraflotation, the wastes also include substantial volumes of carrier (calcite) and other reagent used to float carrier and impurity. For example, a calcite carrier is typically used in amount of about 12 to 20% based on the weight of kaolin flotation feed. The discarded froth product typically contains about 85 to 95% calcite. See U.S. Pat. No. 4,014,709. Early in the development of Ultraflotation a proposal was made to remove the calcite carrier from a kaolin froth product by adding a powerful dispersant (TSPP) to remove reagent from carrier. Although the process did place the carrier in reusable form, it removed collector, therefore necessitating further addition of flotation oil before the material could be reused in Ultraflotation. It would be desirable when treating such a waste by-product to recover the carrier without removing flotation reagents therefrom.